1. Field of the Invention
The present invention relates to an optical amplifying device applied to a wavelength division multiplexing (WDM) communication system.
2. Description of the Related Art
Over the recent years, a transmission capacity of the WDM communication system has sharply increased and, under the actual operation, has reached such a level that signals of a terabit class per core cable (optical fiber) are transmitted.
In the WDM, the optical amplifier amplifies batchwise the signals of a plurality of channels (CH). At this time, if an optical amplification characteristic gets abnormal, all the channels are affected by this abnormality. A fault of the optical amplifier affects tremendously in proportion to the transmission capacity. It is therefore required that deterioration of the characteristic of the optical amplifier is exactly detected as soon as possible and an occurrence point of the fault be specified.
Normally, in a long distance optical transmission system, as shown in FIG. 1, a plurality of optical amplifiers is connected in series at multi-stages between a transmitting side and a receiving side (FIG. 1). Therefore, an abnormal spectrum caused by deterioration of a characteristic of a certain optical amplifier is propagated to a subsequent optical amplifier. Accordingly, even when a signal error is detected in a signal receiving unit such as a transponder on the receiving side, it is impossible to clearly judge which optical amplifier is abnormal.
In the case of an apparent failure such as a stop of the optical output due to a failure of a light-pumped laser, it is easy to notify a monitor control terminal of this abnormality by alarming. It is not, however, easy to detect the abnormality in the case of abnormality of wavelength dependency of a gain of the optical amplifier and in the case of deterioration of a noise characteristic.
An electric current and a temperature of the light-pumped laser within the optical amplifier have hitherto been monitored. Generally, the optical amplifier is, however, constructed of a plurality of light-pumped lasers, and hence it is difficult to clearly gasp a relationship between a degree of deterioration of the individual laser and an amplification characteristic of the optical amplifier.
FIG. 2 shows one example of an abnormal optical spectrum. In FIG. 2, though the electric current etc of the light-pumped laser is normal, noises excessively occur in a short wavelength are a, and the abnormality is seen in the amplification characteristic.
The following Patent document 1 and Patent document 2 are exemplified as documents of the prior art related to the present invention.
[Patent document 1] Japanese Patent Application Laid-Open Publication No. 10-336118
[Patent document 2] Japanese Patent Application Laid-Open Publication No. 2002-368698